This invention relates to a bearing preload measuring method and apparatus adapted to measure the rigidity of a double row bearing or duplex bearing in which a negative gap is formed by a preload.
Heretofore, a double row bearing or a duplex bearing to which a preload is applied by tightening a nut when it is assembled, must be high in rigidity in view of the performance of a machine to which the bearing is applied. However, if an amount of preload is increased thereby to excessively increase the rigidity, then the preload is excessively increased, which lowers the performance of the bearing (the frictional moment is increased, heat is abnormally generated, or a fatigue life is decreased). Hence, the rigidity of the bearing must be controlled in a predetermined range while being related to the amount of preload.
Examples of the bearing preload measuring apparatus, which are known in the art, are as follows: Japanese Patent Unexamined Publication No. 10835/1993 has disclosed a bearing preload measuring apparatus in which a vibration signal generated by a rotating bearing is subjected to frequency analysis, to obtain the contact angle a of the rolling element and the resonance frequency fa, and those data, namely, the contact angle .alpha. and the resonance frequency fa are utilized to obtain a bearing rigidity Ka and an amount of preload Fa.
Japanese Patent Examined Publication No. 61700/1990 has disclosed the following bearing preload measuring apparatus. In the apparatus, the vibration applied to a bearing box or a shaft by a vibrating machine is detected with a speed or acceleration sensor, and the detection signal is amplified by an amplifier and analyzed with a frequency analyzer to detect the resonance frequency of the bearing. The relation between the resonance frequency thus detected and the amount of preload is obtained according to the finite element method in advance, so that the amount of preload is detected from the resonance frequency.
Japanese Patent Unexamined Publication No. 127634/1995 has disclosed the following technique: In assembling a bearing, at the time of temporarily pressing the inner race in the bearing body, a bearing axial gap and a dimension A in the direction of axis between the large-diameter end face of the inner race and the reference surface of an axle are measured. After the inner race has been pressed in the bearing body, the dimension B between the large-diameter end face of the inner race and the reference surface of the axle, whereby the negative bearing axial gap provided after the inner race has been pressed in the bearing body; that is, the initial gap provided before the nut is tightened can be obtained.
The above-described techniques of Japanese Patent Unexamined Publication No. 10835/1993 and Japanese Patent Examined Publication No. 61700/1990 are based on resonance. Hence, if, in the measurement of the rigidity of a duplex bearing, the rigidity is relatively low, then no problem is involved; however, if the rigidity is high, then it is difficult to separate the vibration mode of the structure and the vibration mode due to the elasticity of the bearing from each other. On the other hand, a bearing such as a flanged bearing which has a part low in rigidity suffers from the same problem even if it is a single bearing. In the case of a bearing which must be high in rigidity, it is difficult to accurately measure the rigidity on the basis of the resonance frequency.
The above-described Japanese Patent Unexamined Publication No. 127634/1995 is also disadvantageous in the following point. If, in the case where, by tightening the nut, a preload is applied to the bearing in which the negative gap has been formed, the amount of decrease in gap can be sufficiently controlled, then the measurement of the initial gap is effective. In the case where it is estimated that the amount of gap decrease is fluctuated, it is necessary to evaluate the amount of decrease in gap after the nut is tightened. However, with the method in which the initial gap provided before the nut is tightened is measured, it is difficult to evaluate the amount of decrease in gap after the nut is tightened.